Biodiesel is prepared by a base-catalyzed transesterification of triglycerides such as vegetable oils and animal fats. The transesterification reaction involves reacting triglycerides with an alcohol to form fatty acid alkyl esters and glycerol. The purified esters can be used as biodiesel fuel, whereas glycerol is formed as a byproduct of this process.
Filter plugging and engine failure due to precipitate formation at low temperatures are common problems of vehicles using biodiesel blends. Precipitates can include steryl glucosides, such as phosphorous glucosides, and non-methyl esters, which can increase filtration time and promote crystallization of other compounds contained in the biodiesel. Even at low levels, steryl glucosides can form aggregates with fatty acid methyl esters that may appear as a visible cloud. Aggregates can accelerate filter plugging at a wide range of temperatures due to the high melting point of steryl glucosides. At room temperatures, steryl glucosides can aggregate and plug filters used for biodiesel fuel. At cold temperatures, the cold-flow problems caused by alkyl esters of saturated fatty acids such as monoacylglycerols may be compounded by the presence of steryl glucosides.
Formation of steryl glucosides in biodiesel also may be exacerbated in the presence of trace amounts of water. Unlike alkyl esters of saturated fatty acids, steryl glucosides cannot be easily removed by melting or exposure to heat because the melting point of steryl glucosides is about 240° C. Further, because steryl glucosides are insoluble in many solvents, the cleaning of components having accumulated steryl glucosides can be problematic.
Recent changes in ASTM D6751-08 Standard Specifications for Biodiesel Fuel Blend Stock (B100) include mandatory compliance for biodiesel to meet cold soak filtration specifications. The test subjects biodiesel (B100) to a soak period at cold temperatures, re-warms the biodiesel, and then filters the biodiesel through a 0.7 micron filter, a ASTM D6217 procedure. The test is a qualitative evaluation meant to replicate performance of the biodiesel in cold climates. Developing a cost-effective process for reducing sterol glucosides and other contaminants can result in reduced precipitate formation, and minimize filter plugging problems for automotive fuel delivery systems. By improving its quality, biodiesel can become more competitive on the market, and public acceptance can be enhanced.
A number of processing techniques exist to address removing contaminants, such as ambient filtering, cold filtering, water degummimg, absorbent treatment and vacuum distillation. However, these methods do not provide an energy efficient and cost effective process to remove non-methyl ester impurities and other contaminants from biodiesel fuels and minimize the precipitation problems associated with biodiesel. Accordingly, there is a need for processes for preparing biodiesel fuels and blends able to meet the required purity specifications without forming harmful precipitates that can lead to fuel filter plugging.